A technique for forming and recording a series of light and dark bands in a photorefractive material such as photopolymer is known in the art in which two plane waves of coherent light are emitted and directed into the material so as to interfere with each other thereby producing interference fringes therein so that the bands of the interference fringes are formed in the material as variation in the intensity of light and retained therein as variation in the refractive index of the material (see JP 2005-071557 A, for example). The interference fringes thus recorded in the material may serve as a diffraction grating, and thus the material (interference fringe medium) in which an interference fringe pattern is recorded may be utilized as an optical element or as an information medium in which information is recorded.
It has however been known that the continuous or continual irradiation of photopolymer as required in the aforementioned conventional technique would shrink the material as a result of photopolymerization of the material, so that the interference fringes would disadvantageously be dislocated.
The present invention has been made in an attempt to eliminate this disadvantage. Illustrative, non-limiting embodiments of the present invention overcome the above disadvantage and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantage described above, and illustrative, non-limiting embodiments of the present invention may not overcome the problem described above.